Just Google "Numrich Gun Parts Corporation" and when their website comes up, just type in the "Search" window: "M2 .50 Caliber Browning Machine Gun Parts" and lo and behold, they carry a complete set of parts for the .50 Caliber Machine Guns on your B-17, etc.

They even sell one of those "Training Mats" that has every part of MA Duce pictured on it, so a trainee can take an M2 apart any lay every part over the same picture of that part on the mat!